refactor(grid): change schar_T representation to be more compact

Previously, a screen cell would occupy 28+4=32 bytes per cell
as we always made space for up to MAX_MCO+1 codepoints in a cell.

As an example, even a pretty modest 50*80 screen would consume

50*80*2*32 = 256000, i e a quarter megabyte

With the factor of two due to the TUI side buffer, and even more when
using msg_grid and/or ext_multigrid.

This instead stores a 4-byte union of either:
- a valid UTF-8 sequence up to 4 bytes
- an escape char which is invalid UTF-8 (0xFF) plus a 24-bit index to a
  glyph cache

This avoids allocating space for huge composed glyphs _upfront_, while
still keeping rendering such glyphs reasonably fast (1 hash table lookup
+ one plain index lookup). If the same large glyphs are using repeatedly
on the screen, this is still a net reduction of memory/cache
consumption. The only case which really gets worse is if you blast
the screen full with crazy emojis and zalgo text and even this case
only leads to 4 extra bytes per char.

When only <= 4-byte glyphs are used, plus the 4-byte attribute code,
i e 8 bytes in total there is a factor of four reduction of memory use.
Memory which will be quite hot in cache as the screen buffer is scanned
over in win_line() buffer text drawing

A slight complication is that the representation depends on host byte
order. I've tested this manually by compling and running this
in qemu-s390x and it works fine. We might add a qemu based solution
to CI at some point.

src/nvim/api/vim.c

@@ -1947,7 +1947,9 @@ Array nvim__inspect_cell(Integer grid, Integer row, Integer col, Arena *arena, E
   }
   ret = arena_array(arena, 3);
   size_t off = g->line_offset[(size_t)row] + (size_t)col;
-  ADD_C(ret, CSTR_AS_OBJ((char *)g->chars[off]));
+  char *sc_buf = arena_alloc(arena, MAX_SCHAR_SIZE, false);
+  schar_get(sc_buf, g->chars[off]);
+  ADD_C(ret, CSTR_AS_OBJ(sc_buf));
   int attr = g->attrs[off];
   ADD_C(ret, DICTIONARY_OBJ(hl_get_attr_by_id(attr, true, arena, err)));
   // will not work first time
@@ -1963,6 +1965,11 @@ void nvim__screenshot(String path)
   ui_call_screenshot(path);
 }
 
+void nvim__invalidate_glyph_cache(void)
+{
+  schar_cache_clear_force();
+}
+
 Object nvim__unpack(String str, Error *err)
   FUNC_API_FAST
 {